Drupal's page cache is great while it's populated, but once a page is invalidated (for example when its content changes, or after a cache clear), the next request which required the page to be rebuilt is blocked until the page has been built.
On a busy site, or a site which is being crawled aggressively, this can cause a traffic stampede to build multiple pages, which in turn can overwhelm most web servers.
This module will continue to serve cached pages after they become "stale" (that is, after the content they show has changed, or after the cache has been cleared), but in the background will rebuild the page, without blocking requests.
Background page refreshes happen immediately at the end of the current request, up to a maximum of two (configurable in admin > config > development > performance) concurrent build sessions; if more than this number of stale pages need to be built at the same time, further requests are deferred to a queue which is processed by cron.
